Roast the Eggplant: Preheat your oven to 400°F (200°C) with racks in the upper and lower thirds. Line two large baking sheets with parchment paper. Trim the eggplant ends and slice them lengthwise into 1/3-inch thick planks. Arrange in a single layer, brush both sides with 1/3 cup olive oil, and season with salt and pepper. Roast for 30-35 minutes, swapping pans halfway, until golden and tender. Set aside.
Start the Ragu: While the eggplant roasts, heat 1 tbsp of olive oil in a large Dutch oven over medium heat. Add the onion and cook for 5-7 minutes until soft. Stir in the garlic, oregano, smoked paprika, and red pepper flakes, cooking for 1 minute until fragrant.
Simmer the Ragu: Pour in the crushed tomatoes, cooked lentils, and 1/2 tsp sea salt. Bring to a simmer, then reduce heat to low and cook, partially covered, for at least 20 minutes to thicken. Remove from heat and stir in the fresh basil.
Prepare the Crumble Topping: In a small bowl, combine the panko, chopped walnuts, and Parmesan. Drizzle with 1 tbsp olive oil and toss with your fingers until it resembles coarse, damp sand.
Assemble the Bake: Spread 1 cup of ragu in a 9x13 inch baking dish. Layer with one-third of the eggplant planks, 1.5 cups of sauce, and half the mozzarella. Repeat the layer of eggplant, sauce, and the remaining mozzarella. Finish with a final layer of eggplant and the remaining sauce.
Bake and Rest: Sprinkle the crumble topping evenly over the surface. Bake at 400°F (200°C) for 25-30 minutes, until the sauce is bubbling and the topping is golden brown. Let it rest for 15 minutes before garnishing with basil and serving. This is essential for the layers to set.